Minato used a space-time barrier in chapter 502, and said he has to be careful about where he sends that gigantic blast. If he turned the blast on the Nine-Tailed Fox, he might have destroyed the fox, but Konoha would be leveled to the ground as well.
Why would Itachi say that Madara was, in the past, the strongest Shinobi in the world, if he wasn't? If Minato fought Madara during his prime, Madara would have all his techniques as well, and as I said, a greater control of the Nine-Tailed Fox. Even if the Nine-Tails were out of the way, Madara in his prime would be unbeatable - the only reason Hashirama managed to win was because he turned the Nine-Tailed Fox on Madara, leading to his defeat.
In his weakened state, he has considerably less power, and most go about using other methods to accomplish his plans. But even still, as an empty shell of his former self, Madara manages to put up a fight with the Fourth Hokage, and it is still unclear whether chapter 502 was the end of their battle or not. So I could say the same to you: don't underestimate Madara, because as it looks from here, you're trying to raise Minato to the status of a god in your own way.
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Soon we'll find out what Minato really meant when he said "back then...he saw through everything I did."